[MiyamotoAkira]

Furthermore to Klathmon, there is an opportunity cost. Because you don't have the test, changing that method in the future becomes problematic. Is not the bug reappearing, is the refactoring that cannot longer take place.

[koonsolo]

In practice, the rewrites that I have seen, are not limited to just function bodies, without changing interfaces and class structures.

What I see in refactoring is that whole blocks of code get an overhaul. And in that case, all the test you wrote on those interfaces need to be adapted, rewritten or removed.

So actually, in a proper refactoring, some tests might help you out, because the interface of the classes didn't change. But other tests cost you time.

It's all a pretty complex balancing act. Some tests save you time, some cost you time. They all make the code more stable, but you have to put your effort where it really makes sense.